1. Field of the Invention
The present invention relates to a semiconductor device having epitaxial structures, and more particularly, to a semiconductor device having an epitaxial source/drain.
2. Description of the Prior Art
Epitaxial structures are used in a wide variety of semiconductor applications. For example, the prior art usually forms an epitaxial layer such as a silicon germanium (hereinafter abbreviated as SiGe) layer in a single crystal substrate by performing a selective epitaxial growth (hereinafter abbreviated as SEG) method. Since the epitaxial layer has the crystalline orientation almost identical to the crystalline orientation of the substrate, the epitaxial layers serves as a raised source/drain or a recessed source/drain for the semiconductor device. Because the lattice constant of the epitaxial SiGe layer is larger than that of the silicon substrate, a strain stress is generated to the channel region of the meta-oxide semiconductor (MOS) transistor device. Accordingly, carrier mobility in the channel region is improved and the speed of the MOS transistor is increased.
Although the epitaxial SiGe layer efficiently improves device performance, it increases complexity of the semiconductor fabrication and difficulties of the process control. For example, it is well-known that the stress is increased when the germanium concentration in the SiGe epitaxial structures is increased. However, the germanium concentration cannot be increased as expected: If the thickness of the epitaxial SiGe structure exceeds the critical thickness, it is relaxed and fails to cause stress to the channel region. Except the thickness issue, it is often found that agglomeration is formed by metal and germanium during the silicide process and causes serious junction leakage. Furthermore, the prior art also observes that because the lattice constants in the interface between the SiGe epitaxial structures and the silicon substrate are so different that the threshold voltage (hereinafter abbreviated as Vt) roll-off occurs.
Accordingly, though the epitaxial structure is able to improve the device performance, it is always in need to improve the epitaxial structure itself.